Some possible theoretical variograms are shown in Figure 7 (from Barnes et al., 1977). The horizontal axis represents distance h and the vertical axis the value of the variogram y(h). The variogram in Figure 7(a) is zero at h=0, increases, and then levels off. The point where it levels off, denoted h,, is the range; its interpretation is, that after distance h,, the concentrations are no longer related (correlated). The value yeh ) is called the “sill" and is the variance of the (independent) variables greater than distance h, apart. Figure 7(b) represents a variogram that increases proportional to the distance. This implies that the observations Z(x) and Z(x + h) are related no matter how large the distance h. This is hardly realistic in practice. Figure 7(c) illustrates a type of variogram that appears to be common in radionuclide field studies; y(h) is discontinuous at 0. A variogram must be 0 at h=0 since Z(x) - Z(x) = 0. But Figure 7(c) shows a case where no matter how close two points become, there is still some variability left. The discontinuity of the variogram at h=0 is called the "nugget effect." Finally, Figure 7(d) represents a flat variogram, or pure nugget, meaning the variables appear to be independent regardless of their proximity. As we shall see, variograms of the soil and FIDLER data collected here exhibit some of the features of types a, c, and d. STUDY DESIGN AND RESULTS The data in this study were taken along two transects, one near GZ and the other 3,600 ft north of GZ at the Area 13 site. The approximate location of the two lines is given in Figure 8. Note that Line l (near GZ) crosses several activity level strata, while Line 2 is contained in one of the lowest-level Am strata. The data consists of both surface soil samples analyzed for Am by Ge(Li) counting and FIDLER Am readings in counts per minute. For Line 1, three sets of FIDLER readings and four adjacent ring samples (5-in. diameter and 5-cm deep) were taken every 20 ft. For Line 2, the interval was every 150 ft. There were a total of 15 such clusters of data for each line, spanning about 280 and 2,100 ft for Lines 1 and 2, respectively. The three sets of FIDLER measurements consisted of instrument placed directly on the ring, (1) readings with the (2) readings made with the instrument held 1 ft above the ring, and (3) readings taken at the center of the four samples at 1-ft height. Two 70-g aliquots per ring sample were counted using REECo's Ge(Li) system, giving a total of 120 soil concentrations per line. At a later time, 100 adjacent FIDLER readings, both surface and 1-ft height, were taken at 5-in. intervals 10 ft south of both lines. The purpose was to obtain information on the changes in activity occurring on a scale of less than 40 ft. The complete field sampling and laboratory protocol for this study is given in Appendix C. 425